The present invention is generally directed to fuel cell components, and to solid oxide fuel cell materials in particular.
Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. Electrolyzer cells are electrochemical devices which can use electrical energy to reduce a given material, such as water, to generate a fuel, such as hydrogen. The fuel and electrolyzer cells may comprise reversible cells which operate in both fuel cell and electrolysis mode.
In a high temperature fuel cell system, such as a solid oxide fuel cell (SOFC) system, an oxidizing flow is passed through the cathode side of the fuel cell, while a fuel flow is passed through the anode side of the fuel cell. The oxidizing flow is typically air, while the fuel flow can be a hydrocarbon fuel, such as methane, natural gas, propane, ethanol, or methanol. The fuel cell, operating at a typical temperature between 750° C. and 950° C., enables combination of the oxygen and free hydrogen, leaving surplus electrons behind. The excess electrons are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit.
Fuel cell stacks may be either internally or externally manifolded for fuel and air. In internally manifolded stacks, the fuel and air is distributed to each cell using risers contained within the stack. In other words, the gas flows through openings or holes in the supporting layer of each fuel cell, such as the electrolyte layer, and gas separator of each cell. In externally manifolded stacks, the stack is open on the fuel and air inlet and outlet sides, and the fuel and air are introduced and collected independently of the stack hardware. For example, the inlet and outlet fuel and air flow in separate channels between the stack and the manifold housing in which the stack is located.
Typically, SOFC are fabricated either as electrolyte supported, anode supported, or cathode supported, depending on which of the three functional components of the cell provides structural support. In planar electrolyte supported SOFC designs, the anode and cathode electrodes are painted as an ink onto the opposite surfaces of a planar ceramic electrolyte. However, weakness in structural integrity, particularly at or around the fuel inlet and outlet riser openings, might arise in these designs under certain loading conditions.
Ceramics are known to exhibit fracture before any plastic deformation can occur in response to an applied tensile load, such as during thermal cycling. Fracture in ceramics usually originates at preexisting defects, such as microcracks, internal pores, and grain corners. Fracture strength depends on the probability of the existence of a defect that is capable of initiating a crack. WILLIAM D. CALLISTER, JR., MATERIALS SCIENCE AND ENGINEERING, 5th Ed. (1999), pages 407-409. For two ceramic samples containing identical defect densities, it is known that specimen size and volume influences fracture strength: “the larger the specimen, the greater this flaw existence probability, and the lower the fracture strength.” CALLSISTER, at 408.